Generally, washing machines perform washing operation, rinsing operating, spin-drying operation, and the like through rotation of a rotational tub and a pulsator via driving force of a motor. Upon washing operation, water and laundry in the rotational tub are agitated such that the laundry is washed via friction between the laundry and the tub as well as the water. The washing machines can be classified into pulsator type washing machines, agitator type washing machines, drum type washing machines, and the like according to manners of washing operation.
In particular, the drum type washing machines can be classified into an indirect-coupled type in which driving force of a motor is indirectly transferred to a drum via a belt wound around a motor pulley and a drum pulley, and a direct-coupled type in which the driving force is directly transferred to the drum via a rotor of a BLDC motor which is directly coupled to the drum.
For the indirect-coupled type, as the driving force of the motor is not directly transferred to the drum, but indirectly transferred thereto through the belt wound around the motor pulley and the drum pulley, there are problems in that energy waste and noise generated during transfer of the driving force are excessive.
Currently, there is an increasing tendency of using the direct-coupled type drum washing machine which employs the BLDC motor in order to solve the problems of the conventional drum washing machines.
The construction of the direct-coupled type drum washing machine will be described briefly as one example of conventional washing machines with reference to FIG. 1 as follows.
FIG. 1 is a longitudinal sectional view of the convention drum washing machine which comprises a tub 2 installed within a cabinet 1, and a drum rotatably installed coaxially in the tub 2.
The tub 2 is provided at a rear side with a motor which comprises a stator 6 and a rotor 5. The stator 6 is fixed to a rear wall of the tub 2, and the rotor 5 surrounding the stator 6 penetrates the tub 2 and is then coupled to the drum 3 via the shaft.
FIG. 2 is a partially cut perspective view illustrating a part of the rear wall of the tub.
The rear wall of the tub 2 is provided at its center with a metallic bearing housing 7, which can support bearings positioned on an outer periphery of both ends of a shaft 4, and receives the stator 6 fastened thereto.
The rear wall of the tub 2 is formed with stepped sections 21 and non-stepped sections 22. Each of the stepped sections 21 has a stepped coupling portion 23 formed adjacent to the center of the tub 2 where the bearing housing is positioned, such that the stator 6 is coupled to the coupling portion 23. Each of the non-stepped sections 22 is provided with a reinforcing rib 22a. 
FIG. 3 is a perspective view illustrating the configuration of the stator 6. The stator 6 comprises a ring-shaped core 61 around which a oil 66 is wound, insulators 62 and 63, a hole sensor assembly 64, and a tab housing assembly 65 for connection with a power source.
Here, the insulators 62 and 63 comprise a first insulator 62 and a second insulator 63 formed to encase upper and lower surfaces of the core 6 as shown in the drawing. The insulators 62 and 63 have a plurality of bulges 62a and 63a formed on an inner side thereof so as to define fastening holes 62b and 63b therein, respectively.
The fastening holes 62b and 63b are fastened to the coupling portions 23 on the rear wall of the tub 2 by means of bolts.
Here, the upper surface of the first insulator 62 is formed with a plurality of positioning protrusions 62c adjacent to the bulges 62a to allow the stator 6 to be positioned on the rear wall of the tub when the stator 6 is coupled to the rear wall of the tub. Correspondingly, on the rear wall of the tub 2, the coupling portions 23 are formed with a plurality of positioning holes 23a in which the positioning protrusions 62c will be inserted, respectively.
For the washing machine in which the drum 2 is directly rotated by means of the BLDC motor, the stator 6 is directly mounted to a securing side on the rear wall of the tub 2 as shown in FIG. 4. When considering the weight of the stator 6, vibration thereof upon rotation of high speeds, and shaking and deformation of the rotor 5, a fastening portion between the stator 6 and the tub 2 is required to have a strong structure. In particular, for a large capacity drum washing machine which comprises a stator 6 having a weight of 1.5 kg or more, and has a rotational speed of 600˜2,000 rpm upon spin-drying operation, it is necessary to have a higher structural strength.
Meanwhile, even for other types of washing machine as well as the direct-coupled type washing machine, the rear wall of the tub is required to have high strength and rigidity in the case where the bearing housing is inserted in the rear wall of the tub. The reason is that vibration of a rotational shaft serving to rotate the drum is transferred to the rear wall of the tub via the bearing housing, thus requiring the rear wall of the tub to have a sufficient structural strength.
Thus, if the tub is formed by injection molding with the bearing housing inserted therein, it is desirable that the bearing housing have a flange. The flange is designed to enhance the strength and rigidity of the rear wall of the tub. In addition, the flange serves to enhance coupling force between the bearing housing and the tub.
Accordingly, for the tub formed by the injection molding with the bearing housing having the flange inserted therein, the structure of the flange is very important. When the flange is designed, the strength and rigidity of the rear wall of the tub and the coupling force between the bearing housing and the tub need to be taken into consideration. In addition, since the flange is inserted in the rear wall of the tub, the shape of the flange influences an inner volume of the tub, and as a result, influences the capacity of the washing machine. As such, when constructing the flange, various factors as mentioned above mist be considered.
Further, the structure of the flange also influences manufacture of a mold for the tub as well as manufacture of a mold for the bearing housing. Thus, the flange must be designed to allow easy manufacturing of the mold therefor while redoing costs for manufacturing the mold. That is, the flange must have a simple structure in order to reduce the costs for the mold while allowing easy manufacturing thereof.